An image mask is a function on image coordinates specifying which pixels or regions are retained or weighted. A binary mask selects a region, while a soft mask takes intermediate weights. Applying the mask pointwise gives for image intensity .
For example, a mask equal to one inside a disk and zero outside retains only that disk of an image. An edge with fractional weights can smooth the transition. The mask describes location or weighting, rather than assigning a new color to every retained pixel.
In Fourier analysis, multiplication by a window function in the image plane corresponds to convolution in frequency, with the constant depending on the Fourier convention. A sharp binary boundary can therefore introduce broad high-frequency contributions. This explains why cropping and spectral filtering have different mathematical effects.
New to topics? Read the docs here!